According to the recent trend toward downsizing of electronic devices, inexpensive circuit boards that are capable of mounting semiconductor chips, such as LSIs, are strongly demanded for not only industrial purposes but also for consumer products. It is imperative that such circuit boards having multi-layered fine wiring patterns can be manufactured easily with high yield rate and high reliability, for the purpose of increasing packaging density.
A conventional circuit board includes a glass-epoxy board, which is an insulation substrate made of woven glass cloth impregnated with epoxy resin, and copper foils bonded by heat-press or the like to both sides of the substrate. In circuit board, patterns are formed by photo-etching the copper foils, a through-hole is formed by drilling or the like, and wiring layers between both side surfaces of the through hole are then connected with copper plated on an interior wall of the through-hole.
In this method, an interior of the through hole, upon being plated, lacks reliability because plating solution does not permeate easily into the hole, and an area which is not plated thus tends to be created, and this may cause a failure of electrical connection. This may cause in deficiency that a thickness of the plated copper deep inside the through-hole, which can cause an electrical problem due to a large resistance for an electric connection. It is difficult to mount a component to a portion where a through-hole is formed. And it is difficult to plate a through-hole in a desired inner layer of a multi-layered substrate. These difficulties limit an arrangement of wiring patterns and manufacturing processes of the circuit board, and also hinder downsizing of the board.